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          <h1 class="post-title" itemprop="name headline">JavaScript面向对象编程</h1>
        

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        <h2 id="对象的概念"><a href="#对象的概念" class="headerlink" title="对象的概念"></a>对象的概念</h2><p>其他语言中，对象的两个概念：</p>
<ol>
<li><strong>类</strong>：<strong>类是对象的类型模板</strong>，例如，定义<code>Student</code>类来表示学生，类本身是一种类型，<code>Student</code>表示学生类型，但不表示任何具体的某个学生；</li>
<li><strong>实例</strong>：<strong>实例是根据类创建的对象</strong>，例如，根据<code>Student</code>类可以创建出<code>xiaoming</code>、<code>xiaohong</code>、<code>xiaojun</code>等多个实例，每个实例表示一个具体的学生，他们全都属于<code>Student</code>类型。   </li>
</ol>
<p>所以，类和实例是大多数面向对象编程语言的基本概念。</p>
<h2 id="javaScript中对象的概念"><a href="#javaScript中对象的概念" class="headerlink" title="javaScript中对象的概念"></a>javaScript中对象的概念</h2><p>javaScript不区分类和实例的概念，而是通过原型（prototype）来实现面向对象编程。<br><a id="more"></a><br>原型是指我们要创建<code>xiaoming</code>这个具体的学生时，我们并没有一个<code>Student</code>类型可用。那怎么办？恰好有这么一个现成的对象：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">var robot = &#123;</span><br><span class="line">    name: &apos;Robot&apos;,</span><br><span class="line">    height: 1.6,</span><br><span class="line">    run: function () &#123;</span><br><span class="line">        console.log(this.name + &apos; is running...&apos;);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure></p>
<p>我们看这个<code>robot</code>对象有名字，有身高，还会跑，有点像小明，干脆就根据它来“创建”小明得了！<br>于是我们把它改名为<code>Student</code>，然后创建出<code>xiaoming</code>:<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">var Student = &#123;</span><br><span class="line">    name: &apos;Robot&apos;,</span><br><span class="line">    height: 1.2,</span><br><span class="line">    run: function () &#123;</span><br><span class="line">        console.log(this.name + &apos; is running...&apos;);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line">var xiaoming = &#123;</span><br><span class="line">    name: &apos;小明&apos;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line">xiaoming.__proto__ = Student;</span><br></pre></td></tr></table></figure></p>
<p>注意最后一行代码把<code>xiaoming</code>的原型指向了对象<code>Student</code>，看上去<code>xiaoming</code>仿佛是从<code>Student</code>继承下来的：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">xiaoming.name; // &apos;小明&apos;</span><br><span class="line">xiaoming.run(); // 小明 is running...</span><br></pre></td></tr></table></figure></p>
<p><code>xiaoming</code>有自己的<code>name</code>属性，但并没有定义<code>run()</code>方法。不过，由于小明是从<code>Student</code>继承而来，只要<code>Student</code>有<code>run()</code>方法，<code>xiaoming</code>也可以调用：<br><img src="https://cdn.liaoxuefeng.com/cdn/files/attachments/001435287613668a73ab76ccc85411282c1b1370be41636000/l" alt="image"><br>JavaScript的原型链和Java的Class区别就在，它没有“<strong>Class</strong>”的概念，所有对象都是实例，所谓<strong>继承关系不过是把一个对象的原型指向另一个对象而已</strong>。<br>如果你把<code>xiaoming</code>的原型指向其他对象：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">var Bird = &#123;</span><br><span class="line">    fly: function () &#123;</span><br><span class="line">        console.log(this.name + &apos; is flying...&apos;);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line">xiaoming.__proto__ = Bird;</span><br></pre></td></tr></table></figure></p>
<p>现在<code>xiaoming</code>已经无法<code>run()</code>了，他已经变成了一只鸟：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">xiaoming.fly(); // 小明 is flying...</span><br></pre></td></tr></table></figure></p>
<p>在JavaScrip代码运行时期，你可以把<code>xiaoming</code>从<code>Student</code>变成<code>Bird</code>，或者变成任何对象。  </p>
<blockquote>
<p>请注意，上述代码仅用于演示目的。在编写JavaScript代码时，不要直接用<code>obj.__proto__</code>去改变一个对象的原型，并且，低版本的IE也无法使用<code>__proto__</code>。<code>Object.create()</code>方法可以传入一个原型对象，并创建一个基于该原型的新对象，但是新对象什么属性都没有，因此，我们可以编写一个函数来创建<code>xiaoming</code>：  </p>
</blockquote>
<p>var Student = {// 原型对象<br>    name: ‘Robot’,<br>    height: 1.2,<br>    run: function () {<br>        console.log(this.name + ‘ is running…’);<br>    }<br>};</p>
<p>function createStudent(name) {<br>    // 基于Student原型创建一个新对象:<br>    var s = Object.create(Student);<br>    // 初始化新对象:<br>    s.name = name;<br>    return s;<br>}</p>
<p>var xiaoming = createStudent(‘小明’);<br>xiaoming.run(); // 小明 is running…<br>xiaoming.<strong>proto</strong> === Student; // true</p>
<p><strong>这是创建原型继承的一种方法</strong>  </p>
<h2 id="创建对象"><a href="#创建对象" class="headerlink" title="创建对象"></a>创建对象</h2><blockquote>
<p>JavaScript对每个创建的对象都会设置一个原型，指向它的原型对象。  </p>
</blockquote>
<p>当我们用<code>obj.xxx</code>访问一个对象的属性时，JavaScript引擎先在<strong>当前对象</strong>上查找该属性，如果没有找到，就到<strong>其原型对象</strong>上找，如果还没有找到，就一直上溯到<b><code>Object.prototype</code>对象</b>，最后，如果还没有找到，就只能返回<code>undefined</code>。<br>其原型链是：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">arr ----&gt; Array.prototype ----&gt; Object.prototype ----&gt; null</span><br><span class="line">```  </span><br><span class="line">`Array.prototype`定义了`indexOf()`、`shift()`等方法，因此你可以在所有的`Array`对象上直接调用这些方法。  </span><br><span class="line">当我们创建一个函数时：</span><br></pre></td></tr></table></figure></p>
<p>function foo() {<br>    return 0;<br>}<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">函数也是一个对象，它的原型链是：</span><br></pre></td></tr></table></figure></p>
<p>foo —-&gt; Function.prototype —-&gt; Object.prototype —-&gt; null<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">由于`Function.prototype`定义了`apply()`等方法，因此，所有函数都可以调用`apply()`方法。  </span><br><span class="line">&gt; 很容易想到，如果原型链很长，那么访问一个对象的属性就会因为花更多的时间查找而变得更慢，因此要注意不要把原型链搞得太长。  </span><br><span class="line">### 构造函数</span><br><span class="line">除了直接用`&#123; ... &#125;`创建一个对象外，JavaScript还可以用一种构造函数的方法来创建对象。它的用法是，先定义一个构造函数：</span><br></pre></td></tr></table></figure></p>
<p>function Student(name) {<br>    this.name = name;<br>    this.hello = function () {<br>        alert(‘Hello, ‘ + this.name + ‘!’);<br>    }<br>}<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">你会问，咦，这不是一个普通函数吗？  </span><br><span class="line">这确实是一个普通函数，但是在JavaScript中，可以用&lt;b&gt;关键字`new`&lt;/b&gt;来调用这个函数，并返回一个对象：</span><br></pre></td></tr></table></figure></p>
<p>var xiaoming = new Student(‘小明’);<br>xiaoming.name; // ‘小明’<br>xiaoming.hello(); // Hello, 小明!<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">&gt; 注意，如果不写`new`，这就是一个普通函数，它返回`undefined`。但是，**如果写了`new`，它就变成了一个构造函数，它绑定的this指向新创建的对象，并默认返回`this`，也就是说，不需要在最后写`return this;`。**  </span><br><span class="line"></span><br><span class="line">新创建的`xiaoming`的原型链是：</span><br></pre></td></tr></table></figure></p>
<p>xiaoming —-&gt; Student.prototype —-&gt; Object.prototype —-&gt; null<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">也就是说，`xiaoming`的原型指向函数`Student`的原型。如果你又创建了`xiaohong`、`xiaojun`，那么这些对象的原型与`xiaoming`是一样的：</span><br></pre></td></tr></table></figure></p>
<p>xiaoming ↘<br>xiaohong -→ Student.prototype —-&gt; Object.prototype —-&gt; null<br>xiaojun  ↗<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">用`new Student()`创建的对象还从原型上获得了一个`constructor`属性，它指向函数`Student`本身：</span><br></pre></td></tr></table></figure></p>
<p>xiaoming.constructor === Student.prototype.constructor; // true<br>Student.prototype.constructor === Student; // true</p>
<p>Object.getPrototypeOf(xiaoming) === Student.prototype; // true</p>
<p>xiaoming instanceof Student; // true<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">看晕了吧？用一张图来表示这些乱七八糟的关系就是：</span><br><span class="line">![image](https://cdn.liaoxuefeng.com/cdn/files/attachments/00143529922671163eebb527bc14547ac11363bf186557d000/l)</span><br><span class="line">`Student`创建了一个原型对象，`Student`通过属性`prototype`指向自身的原型对象，`xiaoming`、`xiaohong`也一样指向这个原型对象。这个原型对象自己有个属性`constructor`，可以指向创建自己的`Student`函数本身。</span><br><span class="line">&gt; 红色箭头是原型链。**注意，`Student.prototype`指向的对象就是`xiaoming`、`xiaohong`的原型对象，这个原型对象自己还有个属性`constructor`，指向`Student`函数本身**  </span><br><span class="line"></span><br><span class="line">&gt; 另外，函数`Student`恰好有个属性`prototype`指向`xiaoming`、`xiaohong`的原型对象，但是`xiaoming`、`xiaohong`这些对象可没有prototype这个属性，不过可以用`__proto__`这个非标准用法来查看。 </span><br><span class="line"></span><br><span class="line">不过还有一个小问题，注意观察：</span><br></pre></td></tr></table></figure></p>
<p>xiaoming.name; // ‘小明’<br>xiaohong.name; // ‘小红’<br>xiaoming.hello; // function: Student.hello()<br>xiaohong.hello; // function: Student.hello()<br>xiaoming.hello === xiaohong.hello; // false<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">`xiaoming`和`xiaohong`各自的`name`不同，这是对的，否则我们无法区分谁是谁了。  </span><br><span class="line">`xiaoming`和`xiaohong`各自的`hello`是一个函数，但它们是两个不同的函数，虽然函数名称和代码都是相同的！  </span><br><span class="line">如果我们通过`new Student()`创建了很多对象，这些对象的`hello`函数实际上只需要共享同一个函数就可以了，这样可以节省很多内存。  </span><br><span class="line">要让创建的对象共享一个`hello`函数，根据对象的属性查找原则，我们只要把`hello`函数移动到`xiaoming`、`xiaohong`这些对象共同的原型上就可以了，也就是`Student.prototype`：  </span><br><span class="line">![image](https://cdn.liaoxuefeng.com/cdn/files/attachments/001435299854512faf32868f60348be878982909b5a5d04000/l)  </span><br><span class="line">修改代码如下：</span><br></pre></td></tr></table></figure></p>
<p>function Student(name) {<br>    this.name = name;<br>}</p>
<p>Student.prototype.hello = function () {<br>    alert(‘Hello, ‘ + this.name + ‘!’);<br>};<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">用`new`创建基于原型的JavaScript的对象就是这么简单！  </span><br><span class="line">### 忘记写new怎么办</span><br><span class="line">如果一个函数被定义为用于创建对象的构造函数，但是调用时忘记了写`new`怎么办？  </span><br><span class="line">在strict模式下，`this.name = name`将报错，因为`this`绑定为`undefined`，在非strict模式下，`this.name = name`不报错，因为`this`绑定为`window`，于是无意间创建了全局变量`name`，并且返回`undefined`，这个结果更糟糕。  </span><br><span class="line">所以，调用构造函数千万不要忘记写`new`。为了区分普通函数和构造函数，按照约定，构造函数首字母应当大写，而普通函数首字母应当小写，这样，一些语法检查工具如*jslint*将可以帮你检测到漏写的`new`。  </span><br><span class="line">最后，我们还可以编写一个`createStudent()`函数，在内部封装所有的new操作。一个常用的编程模式像这样：</span><br></pre></td></tr></table></figure></p>
<p>function Student(props) {<br>    this.name = props.name || ‘匿名’; // 默认值为’匿名’<br>    this.grade = props.grade || 1; // 默认值为1<br>}</p>
<p>Student.prototype.hello = function () {<br>    alert(‘Hello, ‘ + this.name + ‘!’);<br>};</p>
<p>function createStudent(props) {<br>    return new Student(props || {})<br>}<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">这个`createStudent()`函数有几个巨大的优点：一是不需要`new`来调用，二是参数非常灵活，可以不传，也可以这么传：  </span><br><span class="line">*props参数传的是对象*</span><br></pre></td></tr></table></figure></p>
<p>var xiaoming = createStudent({<br>    name: ‘小明’<br>});</p>
<p>xiaoming.grade; // 1<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br><span class="line">101</span><br><span class="line">102</span><br><span class="line">103</span><br><span class="line">104</span><br><span class="line">105</span><br><span class="line">106</span><br><span class="line">107</span><br><span class="line">108</span><br><span class="line">109</span><br><span class="line">110</span><br><span class="line">111</span><br><span class="line">112</span><br><span class="line">113</span><br><span class="line">114</span><br><span class="line">115</span><br><span class="line">116</span><br><span class="line">117</span><br><span class="line">118</span><br><span class="line">119</span><br><span class="line">120</span><br><span class="line">121</span><br><span class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br><span class="line">128</span><br><span class="line">129</span><br><span class="line">130</span><br><span class="line">131</span><br><span class="line">132</span><br><span class="line">133</span><br><span class="line">134</span><br><span class="line">135</span><br><span class="line">136</span><br><span class="line">137</span><br><span class="line">138</span><br><span class="line">139</span><br><span class="line">140</span><br><span class="line">141</span><br><span class="line">142</span><br><span class="line">143</span><br><span class="line">144</span><br><span class="line">145</span><br><span class="line">146</span><br><span class="line">147</span><br><span class="line">148</span><br><span class="line">149</span><br><span class="line">150</span><br><span class="line">151</span><br><span class="line">152</span><br><span class="line">153</span><br><span class="line">154</span><br><span class="line">155</span><br><span class="line">156</span><br><span class="line">157</span><br><span class="line">158</span><br><span class="line">159</span><br><span class="line">160</span><br><span class="line">161</span><br><span class="line">162</span><br><span class="line">163</span><br><span class="line">164</span><br><span class="line">165</span><br><span class="line">166</span><br><span class="line">167</span><br></pre></td><td class="code"><pre><span class="line">如果创建的对象有很多属性，我们只需要传递需要的某些属性，剩下的属性可以用默认值。由于参数是一个**Object**，我们无需记忆参数的顺序。如果恰好从`JSON`拿到了一个对象，就可以直接创建出`xiaoming`。</span><br><span class="line">#### 练习</span><br><span class="line">请利用构造函数定义`Cat`，并让所有的`Cat`对象有一个`name`属性，并共享一个方法`say()`，返回字符串`&apos;Hello, xxx!&apos;`：</span><br><span class="line"></span><br><span class="line">    &apos;use strict&apos;;</span><br><span class="line">    function Cat(name)&#123;</span><br><span class="line">        this.name=name||&apos;匿名&apos;;</span><br><span class="line">    &#125;</span><br><span class="line">    Cat.prototype.say=function()&#123;</span><br><span class="line">        return &apos;Hello, &apos;+this.name+&apos;!&apos;;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&gt; 段落总结：如果创建的对象不可以共享的属性例如`name`要放在`Student`函数本身里,这样每次创建新对象都会通过指向的原型对象去复制函数本身的`name`,虽然相像但不是是同一个东西；  </span><br><span class="line">如果是可以共享的属性如`hello`，放在`Student`创建的原型里，每次使用的都是同一个属性。  </span><br><span class="line">## 原型继承</span><br><span class="line">在传统的基于Class的语言如Java、C++中，继承的本质是扩展一个已有的Class，并生成新的Subclass。  </span><br><span class="line">由于这类语言严格区分类和实例，**继承实际上是类型的扩展**。但是，**JavaScript**由于采用**原型继承**，我们无法直接扩展一个Class，**因为根本不存在Class这种类型**。  </span><br><span class="line">但是办法还是有的。我们先回顾`Student`构造函数：</span><br><span class="line"></span><br><span class="line">    function Student(props) &#123;</span><br><span class="line">        this.name = props.name || &apos;Unnamed&apos;;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    Student.prototype.hello = function () &#123;</span><br><span class="line">        alert(&apos;Hello, &apos; + this.name + &apos;!&apos;);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">以及`Student`的原型链：</span><br><span class="line">![Student的原型链](https://cdn.liaoxuefeng.com/cdn/files/attachments/001439872136313496e60e07ed143bda40a0200b12d8cc3000/l)  </span><br><span class="line">现在，我们要基于`Student`扩展出`PrimaryStudent`，可以先定义出`PrimaryStudent`：  </span><br><span class="line"></span><br><span class="line">    function PrimaryStudent(props) &#123;</span><br><span class="line">        // 调用Student构造函数，绑定this变量:</span><br><span class="line">        Student.call(this, props);</span><br><span class="line">        this.grade = props.grade || 1;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">但是，调用了`Student`构造函数不等于继承了`Student`，`PrimaryStudent`创建的对象的原型是：  </span><br><span class="line"></span><br><span class="line">    new PrimaryStudent() ----&gt; PrimaryStudent.prototype ----&gt; Object.prototype ----&gt; null</span><br><span class="line"></span><br><span class="line">必须想办法把原型链修改为：</span><br><span class="line"></span><br><span class="line">    new PrimaryStudent() ----&gt; PrimaryStudent.prototype ----&gt; Student.prototype ----&gt; Object.prototype ----&gt; null</span><br><span class="line"></span><br><span class="line">这样，原型链对了，继承关系就对了。新的基于`PrimaryStudent`创建的对象不但能调用`PrimaryStudent.prototype`定义的方法，也可以调用`Student.prototype`定义的方法。  </span><br><span class="line"></span><br><span class="line">如果你想用最简单粗暴的方法这么干：</span><br><span class="line"></span><br><span class="line">PrimaryStudent.prototype = Student.prototype;</span><br><span class="line"></span><br><span class="line">是不行的！如果这样的话，`PrimaryStudent`和`Student`共享一个原型对象，那还要定义`PrimaryStudent`干啥？</span><br><span class="line"></span><br><span class="line">我们必须借助一个中间对象来实现正确的原型链，这个中间对象的原型要指向`Student.prototype`。为了实现这一点，参考道爷（就是发明JSON的那个道格拉斯）的代码，中间对象可以用一个空函数F来实现：  </span><br><span class="line"></span><br><span class="line">    // PrimaryStudent构造函数:</span><br><span class="line">    function PrimaryStudent(props) &#123;</span><br><span class="line">        Student.call(this, props);</span><br><span class="line">        this.grade = props.grade || 1;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    // 空函数F:</span><br><span class="line">    function F() &#123;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    // 把F的原型指向Student.prototype:</span><br><span class="line">    F.prototype = Student.prototype;</span><br><span class="line"></span><br><span class="line">    // 把PrimaryStudent的原型指向一个新的F对象，F对象的原型正好指向Student.prototype:</span><br><span class="line">    PrimaryStudent.prototype = new F();</span><br><span class="line"></span><br><span class="line">    // 把PrimaryStudent原型的构造函数修复为PrimaryStudent:</span><br><span class="line">    PrimaryStudent.prototype.constructor = PrimaryStudent;</span><br><span class="line"></span><br><span class="line">    // 继续在PrimaryStudent原型（就是new F()对象）上定义方法：</span><br><span class="line">    PrimaryStudent.prototype.getGrade = function () &#123;</span><br><span class="line">        return this.grade;</span><br><span class="line">    &#125;;</span><br><span class="line"></span><br><span class="line">    // 创建xiaoming:</span><br><span class="line">    var xiaoming = new PrimaryStudent(&#123;</span><br><span class="line">        name: &apos;小明&apos;,</span><br><span class="line">        grade: 2</span><br><span class="line">    &#125;);</span><br><span class="line">    xiaoming.name; // &apos;小明&apos;</span><br><span class="line">    xiaoming.grade; // 2</span><br><span class="line"></span><br><span class="line">    // 验证原型:</span><br><span class="line">    xiaoming.__proto__ === PrimaryStudent.prototype; // true</span><br><span class="line">    xiaoming.__proto__.__proto__ === Student.prototype; // true</span><br><span class="line"></span><br><span class="line">    // 验证继承关系:</span><br><span class="line">    xiaoming instanceof PrimaryStudent; // true</span><br><span class="line">    xiaoming instanceof Student; // true</span><br><span class="line"></span><br><span class="line">用一张图来表示新的原型链：</span><br><span class="line">![新的原型链](https://cdn.liaoxuefeng.com/cdn/files/attachments/001439872160923ca15925ec79f4692a98404ddb2ed5503000/l)</span><br><span class="line">注意，函数`F`仅用于桥接，我们仅创建了一个`new F()`实例，而且，没有改变原有的`Student`定义的原型链。</span><br><span class="line"></span><br><span class="line">如果把继承这个动作用一个`inherits()`函数封装起来，还可以隐藏`F`的定义，并简化代码：</span><br><span class="line"></span><br><span class="line">    function inherits(Child, Parent) &#123;</span><br><span class="line">        var F = function () &#123;&#125;;</span><br><span class="line">        F.prototype = Parent.prototype;</span><br><span class="line">        Child.prototype = new F();</span><br><span class="line">        Child.prototype.constructor = Child;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    这个`inherits()`函数可以复用： </span><br><span class="line"></span><br><span class="line">    function Student(props) &#123;</span><br><span class="line">        this.name = props.name || &apos;Unnamed&apos;;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    Student.prototype.hello = function () &#123;</span><br><span class="line">        alert(&apos;Hello, &apos; + this.name + &apos;!&apos;);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    function PrimaryStudent(props) &#123;</span><br><span class="line">        Student.call(this, props);</span><br><span class="line">        this.grade = props.grade || 1;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    // 实现原型继承链:</span><br><span class="line">    inherits(PrimaryStudent, Student);</span><br><span class="line"></span><br><span class="line">    // 绑定其他方法到PrimaryStudent原型:</span><br><span class="line">    PrimaryStudent.prototype.getGrade = function () &#123;</span><br><span class="line">        return this.grade;</span><br><span class="line">    &#125;;</span><br><span class="line"></span><br><span class="line">#### 小结</span><br><span class="line">JavaScript的原型继承实现方式就是：</span><br><span class="line">1. 定义新的构造函数，并在内部用`call()`调用希望“继承”的构造函数，并绑定`this`；</span><br><span class="line">2. 借助中间函数F实现原型链继承，最好通过封装的`inherits`函数完成；</span><br><span class="line">3. 继续在新的构造函数的原型上定义新方法。</span><br><span class="line">## class继承  </span><br><span class="line">&gt; 在上面的章节中我们看到了JavaScript的对象模型是基于原型实现的，特点是简单，缺点是理解起来比传统的类－实例模型要困难，最大的缺点是继承的实现需要编写大量代码，并且需要正确实现原型链。  </span><br><span class="line"></span><br><span class="line">有没有更简单的写法？有！</span><br><span class="line"></span><br><span class="line">新的关键字`class`从ES6开始正式被引入到JavaScript中。`class`的目的就是让定义类更简单。</span><br><span class="line"></span><br><span class="line">我们先回顾用函数实现`Student`的方法：</span><br><span class="line"></span><br><span class="line">    function Student(name) &#123;</span><br><span class="line">        this.name = name;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    Student.prototype.hello = function () &#123;</span><br><span class="line">        alert(&apos;Hello, &apos; + this.name + &apos;!&apos;);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">如果用新的`class`关键字来编写`Student`，可以这样写：  </span><br><span class="line"></span><br><span class="line">    class Student &#123;</span><br><span class="line">        constructor(name) &#123;</span><br><span class="line">            this.name = name;</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        hello() &#123;</span><br><span class="line">            alert(&apos;Hello, &apos; + this.name + &apos;!&apos;);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">比较一下就可以发现，`class`的定义包含了构造函数`constructor`和定义在原型对象上的函数`hello()`（*注意没有function关键字*），这样就避免了`Student.prototype.hello = function () &#123;...&#125;`这样分散的代码。  </span><br><span class="line">最后，创建一个`Student`对象代码和前面章节完全一样：</span><br></pre></td></tr></table></figure></p>
<p>var xiaoming = new Student(‘小明’);<br>xiaoming.hello();<br><code>`</code></p>
<h3 id="class继承"><a href="#class继承" class="headerlink" title="class继承"></a>class继承</h3><p>用<code>class</code>定义对象的另一个巨大的好处是继承更方便了。想一想我们从<code>Student</code>派生一个<code>PrimaryStudent</code>需要编写的代码量。现在，原型继承的中间对象，原型对象的构造函数等等都不需要考虑了，直接通过<code>extends</code>来实现：  </p>
<pre><code>class PrimaryStudent extends Student {
    constructor(name, grade) {
        super(name); // 记得用super调用父类的构造方法!
        this.grade = grade;
    }

    myGrade() {
        alert(&apos;I am at grade &apos; + this.grade);
    }
}
</code></pre><blockquote>
<p>注意<code>PrimaryStudent</code>的定义也是<code>class</code>关键字实现的，而<code>extends</code>则表示原型链对象来自<code>Student</code>。子类的构造函数可能会与父类不太相同，例如，<code>PrimaryStudent</code>需要<code>name</code>和<code>grade</code>两个参数，并且需要通过<code>super(name)</code>来调用父类的构造函数，否则父类的<code>name</code>属性无法正常初始化。</p>
</blockquote>
<p><code>PrimaryStudent</code>已经自动获得了父类<code>Student</code>的<code>hello</code>方法，我们又在子类中定义了新的<code>myGrade</code>方法。  </p>
<p>ES6引入的<code>class</code>和原有的<code>JavaScript</code>原型继承有什么区别呢？实际上它们没有任何区别，<code>class</code>的作用就是让<code>JavaScript</code>引擎去实现原来需要我们自己编写的原型链代码。简而言之，用class的好处就是极大地简化了原型链代码。  </p>
<h4 id="练习"><a href="#练习" class="headerlink" title="练习"></a>练习</h4><p>请利用<code>class</code>重新定义<code>Cat</code>，并让它从已有的<code>Animal</code>继承，然后新增一个方法<code>say()</code>，返回字符串<code>&#39;Hello, xxx!&#39;</code>：</p>
<pre><code>&apos;use strict&apos;;

class Animal {
    constructor(name) {
        this.name = name;
    }
}
class Cat extends Animal{
  constructor(name){
    super(name);
  }
  say(){
    return `Hello, ${this.name}!`;
  }
}
</code></pre><h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>无论是ES5还是ES6还都是使用<code>prototype</code>原型继承的方式来模拟其他语言中类和实例的方式来实现原型继承。<br><code>class</code>只是把代码抽象一下，从语言层面看上去跟其他语言差不多了，背后并没有改变。</p>
<p>不过日常工作没有关系，尽情使用class来实现面向对象编程好了。背后的处理交给工程化工具。</p>
<blockquote>
<p>基本照抄廖雪峰老师的文章，插入了一些自己的感想。<a href="https://www.liaoxuefeng.com/wiki/001434446689867b27157e896e74d51a89c25cc8b43bdb3000/001434499763408e24c210985d34edcabbca944b4239e20000" target="_blank" rel="noopener">原文地址</a></p>
</blockquote>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-2"><a class="nav-link" href="#对象的概念"><span class="nav-number">1.</span> <span class="nav-text">对象的概念</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#javaScript中对象的概念"><span class="nav-number">2.</span> <span class="nav-text">javaScript中对象的概念</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#创建对象"><span class="nav-number">3.</span> <span class="nav-text">创建对象</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#class继承"><span class="nav-number">3.1.</span> <span class="nav-text">class继承</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#练习"><span class="nav-number">3.1.1.</span> <span class="nav-text">练习</span></a></li></ol></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#总结"><span class="nav-number">4.</span> <span class="nav-text">总结</span></a></li></ol></div>
            

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    var isfetched = false;
    var isXml = true;
    // Search DB path;
    var search_path = "search.xml";
    if (search_path.length === 0) {
      search_path = "search.xml";
    } else if (/json$/i.test(search_path)) {
      isXml = false;
    }
    var path = "/" + search_path;
    // monitor main search box;

    var onPopupClose = function (e) {
      $('.popup').hide();
      $('#local-search-input').val('');
      $('.search-result-list').remove();
      $('#no-result').remove();
      $(".local-search-pop-overlay").remove();
      $('body').css('overflow', '');
    }

    function proceedsearch() {
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay"></div>')
        .css('overflow', 'hidden');
      $('.search-popup-overlay').click(onPopupClose);
      $('.popup').toggle();
      var $localSearchInput = $('#local-search-input');
      $localSearchInput.attr("autocapitalize", "none");
      $localSearchInput.attr("autocorrect", "off");
      $localSearchInput.focus();
    }

    // search function;
    var searchFunc = function(path, search_id, content_id) {
      'use strict';

      // start loading animation
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay">' +
          '<div id="search-loading-icon">' +
          '<i class="fa fa-spinner fa-pulse fa-5x fa-fw"></i>' +
          '</div>' +
          '</div>')
        .css('overflow', 'hidden');
      $("#search-loading-icon").css('margin', '20% auto 0 auto').css('text-align', 'center');

      $.ajax({
        url: path,
        dataType: isXml ? "xml" : "json",
        async: true,
        success: function(res) {
          // get the contents from search data
          isfetched = true;
          $('.popup').detach().appendTo('.header-inner');
          var datas = isXml ? $("entry", res).map(function() {
            return {
              title: $("title", this).text(),
              content: $("content",this).text(),
              url: $("url" , this).text()
            };
          }).get() : res;
          var input = document.getElementById(search_id);
          var resultContent = document.getElementById(content_id);
          var inputEventFunction = function() {
            var searchText = input.value.trim().toLowerCase();
            var keywords = searchText.split(/[\s\-]+/);
            if (keywords.length > 1) {
              keywords.push(searchText);
            }
            var resultItems = [];
            if (searchText.length > 0) {
              // perform local searching
              datas.forEach(function(data) {
                var isMatch = false;
                var hitCount = 0;
                var searchTextCount = 0;
                var title = data.title.trim();
                var titleInLowerCase = title.toLowerCase();
                var content = data.content.trim().replace(/<[^>]+>/g,"");
                var contentInLowerCase = content.toLowerCase();
                var articleUrl = decodeURIComponent(data.url);
                var indexOfTitle = [];
                var indexOfContent = [];
                // only match articles with not empty titles
                if(title != '') {
                  keywords.forEach(function(keyword) {
                    function getIndexByWord(word, text, caseSensitive) {
                      var wordLen = word.length;
                      if (wordLen === 0) {
                        return [];
                      }
                      var startPosition = 0, position = [], index = [];
                      if (!caseSensitive) {
                        text = text.toLowerCase();
                        word = word.toLowerCase();
                      }
                      while ((position = text.indexOf(word, startPosition)) > -1) {
                        index.push({position: position, word: word});
                        startPosition = position + wordLen;
                      }
                      return index;
                    }

                    indexOfTitle = indexOfTitle.concat(getIndexByWord(keyword, titleInLowerCase, false));
                    indexOfContent = indexOfContent.concat(getIndexByWord(keyword, contentInLowerCase, false));
                  });
                  if (indexOfTitle.length > 0 || indexOfContent.length > 0) {
                    isMatch = true;
                    hitCount = indexOfTitle.length + indexOfContent.length;
                  }
                }

                // show search results

                if (isMatch) {
                  // sort index by position of keyword

                  [indexOfTitle, indexOfContent].forEach(function (index) {
                    index.sort(function (itemLeft, itemRight) {
                      if (itemRight.position !== itemLeft.position) {
                        return itemRight.position - itemLeft.position;
                      } else {
                        return itemLeft.word.length - itemRight.word.length;
                      }
                    });
                  });

                  // merge hits into slices

                  function mergeIntoSlice(text, start, end, index) {
                    var item = index[index.length - 1];
                    var position = item.position;
                    var word = item.word;
                    var hits = [];
                    var searchTextCountInSlice = 0;
                    while (position + word.length <= end && index.length != 0) {
                      if (word === searchText) {
                        searchTextCountInSlice++;
                      }
                      hits.push({position: position, length: word.length});
                      var wordEnd = position + word.length;

                      // move to next position of hit

                      index.pop();
                      while (index.length != 0) {
                        item = index[index.length - 1];
                        position = item.position;
                        word = item.word;
                        if (wordEnd > position) {
                          index.pop();
                        } else {
                          break;
                        }
                      }
                    }
                    searchTextCount += searchTextCountInSlice;
                    return {
                      hits: hits,
                      start: start,
                      end: end,
                      searchTextCount: searchTextCountInSlice
                    };
                  }

                  var slicesOfTitle = [];
                  if (indexOfTitle.length != 0) {
                    slicesOfTitle.push(mergeIntoSlice(title, 0, title.length, indexOfTitle));
                  }

                  var slicesOfContent = [];
                  while (indexOfContent.length != 0) {
                    var item = indexOfContent[indexOfContent.length - 1];
                    var position = item.position;
                    var word = item.word;
                    // cut out 100 characters
                    var start = position - 20;
                    var end = position + 80;
                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
                      return sliceRight.searchTextCount - sliceLeft.searchTextCount;
                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
                      return sliceLeft.start - sliceRight.start;
                    }
                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
                    slicesOfContent = slicesOfContent.slice(0, upperBound);
                  }

                  // highlight title and content

                  function highlightKeyword(text, slice) {
                    var result = '';
                    var prevEnd = slice.start;
                    slice.hits.forEach(function (hit) {
                      result += text.substring(prevEnd, hit.position);
                      var end = hit.position + hit.length;
                      result += '<b class="search-keyword">' + text.substring(hit.position, end) + '</b>';
                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-frown-o fa-5x" /></div>'
            } else {
              resultItems.sort(function (resultLeft, resultRight) {
                if (resultLeft.searchTextCount !== resultRight.searchTextCount) {
                  return resultRight.searchTextCount - resultLeft.searchTextCount;
                } else if (resultLeft.hitCount !== resultRight.hitCount) {
                  return resultRight.hitCount - resultLeft.hitCount;
                } else {
                  return resultRight.id - resultLeft.id;
                }
              });
              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
                searchResultList += result.item;
              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
            }
          }

          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
              if (event.keyCode === 13) {
                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
      if (shouldDismissSearchPopup) {
        onPopupClose();
      }
    });
  </script>





  

  

  
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